How The Scale Was Derived

The tests were arranged in order of difficulty, as found by trying them upon some
200 normal children of different ages from 3 to 15 years. It was found, for illustration,
that a certain test was passed by only a very small proportion of the younger
children, say the 5-year-olds, and that the number passing this test
increased rapidly in the succeeding years until by the age of 7 or
8 years, let us say, practically all the children were successful.
If, in our supposed case, the test was passed by about two thirds to
three fourths of the normal children aged 7 years, it was considered by
Binet a test of 7-year intelligence. In like manner, a test passed by
65 to 75 per cent of the normal 9-year-olds was considered a test of
9-year intelligence, and so on. By trying out many different tests in
this way it was possible to secure five tests to represent each age from
3 to 10 years (excepting age 4, which has only four tests), five for
age 12, five for 15, and five for adults, making 54 tests in all.